Common sense would tell me, as a new bride, that unless I wanted to "go to war" with my in-laws, I don't give their wedding gifts away.
However, there is no real proof that it was given away, so MIL should simply ask did they use it during the past winter, and was it warm, maybe offer a few hints on storing it. When asking about whether it was warm or not, she could say something like, "With every stitch in the quilt I included a million wishes for your happiness. I really hope you enjoy it."